Don't remove existing options when filling #d2l-courses
Don't remove existing options when filling #d2l-courses

When adding options to select#d2l-courses from the enrollments returned
by /d2l/api/lp/1.4/enrollments/myenrollments/, don’t remove existing
options in the select element. This is to allow for multiple calls to
myenrollments when the user has more enrollments than can be returned
in a single call.

This change assumes that select#d2l-courses is empty by default.

ucalgary/d2l-grades-export,ucalgary/d2l-grades-export